1. Field of the Invention
The invention relates to centrifugal fans and impellers, and in particular to centrifugal fans and impellers with two centrifugal blade sets.
2. Description of the Related Art
Conventional centrifugal fans are often integrally formed by using a single mold. When the summation of the blades of an impeller of a centrifugal fan is increased, the thickness of each blade is reduced because of the required thickness of the mold. Thus, the blades are easily deformed or damaged during operation of the centrifugal fan. To maintain sufficient thickness of the blades, the thickness of the mold needs to be significantly reduced. The mold, however, is easily broken during formation of the blades.
Accordingly, the summation of the blades of the impeller of the centrifugal fan cannot be increased, such that wind pressure and wind volume provided by the centrifugal fan are limited. To increase the wind pressure and wind volume provided by the centrifugal fan, rotational speed thereof must be increased. However, noise generated by the centrifugal fan is increased according to the rotational speed. Moreover, as the blades of the impeller of the conventional centrifugal fan have the same shape, resonant noise is also generated during operation of the conventional centrifugal fan.
Hence, there is a need for centrifugal fans and impellers having more blades.